<p>Carbonaceous mudstone is a potential filling material for highway embankments in areas rich in this resource, as its utilization can help reduce environmental impacts. This study aims to investigate the mechanical behavior and particle breakage mechanisms of embankment filler composed of carbonaceous mudstone with different coarse particle contents using discrete element methods. The carbonaceous mudstone particles were drawn according to their real shapes using vector software and then imported into PFC by the FISH language and constructed as clumps, with the shell serving as the specimen wall. The microscopic parameters of the PFC model for numerical triaxial tests were calibrated using data from laboratory triaxial tests. Subsequently, the mechanical behavior (i.e., the stress-strain curve, shear strength, force chain distribution, and displacement process) and the particle breakage (i.e., morphology, quantification, and mechanism) of coarse carbonaceous mudstone particle filler were analyzed. The results show that the shear strength and its parameters of carbonaceous mudstone filler are non-linearly correlated to the coarse particle content. Most force chains concentrate near coarse particles, forming a skeleton force chain phenomenon. The relative breakage rate of carbonaceous mudstone particles is positively correlated with the confining pressure and coarse particle content. As the coarse particle content increases, the deformation of the filler shifts from pore collapses to a combination of pore collapses and particle breakages, and ultimately to particle breakages alone. A coarse particle content between 60% and 80% is recommended for highway embankments constructed with carbonaceous mudstone filler.</p>